66 THE GLACIAL LAKE AGASSIZ. 



THE ARCHEAN AREA IN MINNESOTA. 



The most southwestern outcrops of Archeau formations in Minnesota 

 are 10 to 20 miles southwest of the Minnesota River in Redwood and 

 Yellow Medicine counties, where small isolated exposures of granite, gneiss, 

 and schists occur. The deeply eroded valley of the Minnesota River, 

 channeled by the River Warren, outflowing from Lake Agassiz, cuts through 

 the drift sheet to the bed-rocks, wliich from Big Stone Lake to Little Rock 

 Creek, 4 miles below Fort Ridgely, are Archean gneisses, varying from a 

 granitoid to a schistose structure. In the next 13 miles no rocks older 

 than the Cretaceous are found. Then comes the last Archean outcrop, a 

 coarse granite, opposite to the southeast part of New Uhn, succeeded east- 

 ward by Algonkian conglomerate and quartzite. Observations of the strike 

 and dip of the Archean rocks exposed in this valley show that the axial 

 lines of their folds run mainly from southwest to northeast. 



Central Minnesota has frequent Archeau outcrops in Stearns, Benton, 

 and Momson counties, including the valuable quarries of St. Cloud, Sauk 

 Rapids, and Watab. The greater part of this area is hornbleudic granite, 

 and exhibits no laminated or gneissic structure. It has considerable variety 

 of texture as to its coarseness of gi'ain and readiness to be quarried and 

 wrought into any required form. Mostly its color is light-gray, l^ut upon 

 some extensive tracts it has a red tint similar to that of the celebrated 

 granite of Aberdeen, in Scotland. In other portions of this district, mica- 

 ceous granite, gneiss, and mica-schist are the common rocks, sometimes 

 associated with hornblendic granite. Their sti'ike is usually to the north- 

 east or east-northeast. At Little Falls and Pike Rapids, on the Mississippi, 

 and for several miles to the south, west, and north, as also in northern Todd 

 County, and along'the falls of the St. Louis above Fond du Lac, and thence 

 northeastward, is a group of rocks quite different from the foregoing, its 

 range of variation being from highly cleavable clay-slate, and from mica- 

 schist, inclosing many crystals of staurolite and sometimes garnet and iron 

 pyrites, to very compact, tough, and massive diorite. Comparing these 

 rocks with the divisions of the Archean recognized in Canada and elsewhere, 

 tlie granites and gneisses appear to represent the Laureutian, while the 

 slate, staurolitic schist, and diorite are i)rt>l)al3h' Keewatin. 



